The Mysterious Sarcophagus is an ancient Egyptian engraving block that exclusively spawns in the middle of Anu's Castle. It holds the hibernating Anu, who waits patiently for victims to come looking for him in his Castle. It is completely unbreakable and unobtainable unless creative mode is used. It was added in the 7.2 'Ancient Kings' Build.
The Mysterious Sarcophagus is a large box, at roughly 2.8 blocks tall. It is black with several small yellow and blue rings around it. It has a large carving of Anu's face at the top of it.
To activate the Sarcophagus and free Anu, the player must first reach it by building a staircase or making a tower. Then, they must right click it with a Scarab Gem. The gem will then be imbedded inside the Sarcophagus. You must right click it once more to recieve your Scarab back, and to open the Sarcophagus, freeing Anu, and thus starting the boss battle (see Anu's page for the battle).